Stable blue phosphorescent organic light emitting devices

ABSTRACT

Novel combination of materials and device architectures for organic light emitting devices are provided. In some aspects, specific charge carriers and solid state considerations are features that may result in a device having an unexpectedly long lifetime. In some aspects, emitter purity is a feature that may result in devices having unexpectedly long lifetime. In some aspects, structural and optical considerations are features that may result in a device having an unexpectedly long lifetime. In some aspects, an emissive layer including an organic phosphorescent emissive dopant and an organic carbazole host material results in devices having an unexpectedly long lifetime.

This application claims priority to U.S. Provisional Application Ser.No. 60/986,711, filed Nov. 9, 2007, the disclosure of which is hereinexpressly incorporated by reference.

The claimed invention was made by, on behalf of, and/or in connectionwith one or more of the following parties to a joint universitycorporation research agreement: Princeton University, The University ofSouthern California, and the Universal Display Corporation. Theagreement was in effect on and before the date the claimed invention wasmade, and the claimed invention was made as a result of activitiesundertaken within the scope of the agreement.

BACKGROUND

1. Field of the Invention

The invention relates generally relates to organic light emittingdevices (OLEDs). More specifically, the invention is directed to novelcombination of materials and device architectures that result inexceptionally long-lived blue devices.

2. Related Art

Opto-electronic devices that make use of organic materials are becomingincreasingly desirable for a number of reasons. Many of the materialsused to make such devices are relatively inexpensive, so organicopto-electronic devices have the potential for cost advantages overinorganic devices. In addition, the inherent properties of organicmaterials, such as their flexibility, may make them well suited forparticular applications such as fabrication on a flexible substrate.Examples of organic opto-electronic devices include organic lightemitting devices (OLEDs), organic phototransistors, organic photovoltaiccells, and organic photodetectors. For OLEDs, the organic materials mayhave performance advantages over conventional materials. For example,the wavelength at which an organic emissive layer emits light maygenerally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage isapplied across the device. OLEDs are becoming an increasinglyinteresting technology for use in applications such as flat paneldisplays, illumination, and backlighting. Several OLED materials andconfigurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full colordisplay. Industry standards for such a display call for pixels adaptedto emit particular colors, referred to as “saturated” colors. Inparticular, these standards call for saturated red, green, and bluepixels. Color may be measured using 1931 CIE coordinates, which are wellknown to the art.

One example of a green emissive molecule is tris(2-phenylpyridine)iridium, denoted Ir(ppy)₃, which has the structure of Formula I:

In this, and later figures herein, the dative bond from nitrogen tometal (here, Ir) as a straight line is depicted.

The limited operational stability of organic light emitting devices(OLEDs), however, presents a challenge to their wide-spread acceptancefor use in large-area displays and solid-state lighting. While improvedpackaging techniques and material purity have lead to significantprogress in eliminating extrinsic sources of degradation, the remainingintrinsic luminance loss and voltage rise accompanying long term deviceoperation are not yet well understood.

Various hypotheses have been offered to explain the basis for intrinsicdegradation in device efficiency, with the most widely acceptedadvocating chemical degradation of a fraction of the emissiveconstituent molecules. Presumably, bond cleavage produces radicalfragments, which then participate in further radical addition reactionsto form even more degradation products. These products act asnon-radiative recombination centers, luminescence quenchers, and deepcharge traps. For example, several studies have shown that both anionsand cations of tris(8-hydroxyquinoline) aluminum (Alg₃) are unstable,and evidence has recently been presented that the excited statesthemselves may form reaction centers in the case of the common hostmaterial 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP).

SUMMARY OF THE INVENTION

The invention provides an organic light emitting device that may providenumerous advantages over devices currently in use. The organic lightemitting device including an anode, a cathode, and an emissive layerdisposed between the anode and the cathode. The invention may beimplemented in a number of ways.

In one aspect, charge carriers and solid state considerations arefeatures that may result in a device having a longer lifetime. A devicemay have an emissive layer that includes a host having a triplet energyand a phosphorescent emissive dopant having a peak emissive wavelengthless than 500 nm. The device also includes an exciton blocking layer atleast 5 nm thick disposed between the emissive layer and the cathode andadjacent to the emissive layer, the exciton blocking layer consistingessentially of materials having a triplet energy greater than or equalto the triplet energy of the host of the emissive layer. The emissivelayer is at least 40 nm thick. Electrons in the emissive layer arecarried predominantly by the host. The HOMO of the phosphorescentemissive dopant is at least 0.5 eV higher than the HOMO of the host. Theconcentration of the phosphorescent emissive dopant in the emissivelayer is at least 9 wt %. The phosphorescent emissive dopant comprises acyclometallated N,C-donor imidazophenanthridine ligand comprising atleast one 2,2,2-trialklethyl substituent. It is believed that thiscombination of features leads to a device having an unexpectedly longlifetime.

In one aspect, emitter purity is a feature that may result in deviceshaving longer lifetime. In particular, a device that includes, interalia, an emissive layer including a host and a phosphorescent emissivedopant having a peak emissive wavelength less than 500 nm and where (i)the phosphorescent emissive dopant is deposited from a source that has apurity in excess of about 99.5% as determined by high pressure liquidchromatography, (ii) the source further having a combined concentrationof halide and metal impurities below about 100 ppm, (iii) thephosphorescent emissive dopant leaves a residue corresponding to lessthan about 5 wt % of the original charge in the sublimation crucible,and (iv) the phosphorescent emissive dopant has a sublimationtemperature less than about 350° C. and is deposited via sublimation mayhave a surprisingly long lifetime. The phosphorescent emissive dopantmay be deposited in an environment having less than 10 ppm oxygen and inthe near absence of light.

In one aspect, charge carriers and solid state considerations arefeatures that may result in a device having a longer lifetime. Inparticular, a device including an emissive layer disposed between theanode and the cathode, such that the emissive layer includes a host anda phosphorescent emissive dopant having a peak emissive wavelength lessthan about 500 nm and such that when a voltage is applied across thedevice, electrons in the emissive layer are carried predominantly by thehost and where the (i) HOMO of the phosphorescent dopant is at leastabout 0.5 eV higher than the HOMO of the host and (ii) thephosphorescent emissive dopant has a concentration of at least about 9wt % in the emissive layer may have a surprisingly longer lifetime.

In one aspect, structural and optical considerations are features thatmay result in a device having a longer lifetime. For example, a deviceincluding a CuPc hole injection layer disposed between the anode and thecathode and adjacent to the anode, an emissive layer disposed betweenthe anode and the cathode such that the emissive layer includes a hostand a phosphorescent emissive dopant having a peak emissive wavelengthless than about 500 nm, and an exciton blocking at least about 5 nmthick disposed between the emissive layer and the cathode and adjacentto the emissive layer such that the exciton blocking layer consistingessentially of materials having a triplet energy greater than or equalto the host of the emissive layer and where (i) the emissive layer is atleast about 40 nm thick, and (ii) the emissive layer is at least about40 nm from the cathode may have a surprisingly longer lifetime.

In one aspect, the device may include a CuPc hole injection layerdisposed between the anode and the cathode and adjacent to the anode, anemissive layer disposed between the anode and the cathode such that thatthe emissive layer includes a host and a phosphorescent emissive dopanthaving a peak emissive wavelength less than about 500 nm, and an excitonblocking layer at least about 5 nm thick disposed between the emissivelayer and the cathode and adjacent to the emissive layer such that theexciton blocking layer consists essentially of materials having atriplet energy greater than or equal to the triplet energy of the hostof the emissive layer and where (i) the emissive layer is at least 40 nmthick; and (ii) the surface plasmon mode is less than about 30% may havea longer lifetime.

In one aspect, a device is provided having an anode, a cathode, and anemissive layer disposed between the anode and the cathode. The emissivelayer includes an organic phosphorescent emissive dopant, and an organiccarbazole host material.

Preferably, an ultra high vacuum system may be used in addition to theparticular combinations of device lifetime-enhancing features, asdisclosed herein. In particular, depositing organic materials (e.g.,phosphorescent emissive dopant) in an ultra high vacuum system having apressure level less than about 1×10⁻⁸ Torr, preferably in the range ofabout 1×10⁻⁸ Torr to about 1×10⁻¹² Torr, more preferably in the range ofabout 1×10⁻⁹ Torr to about 1×10⁻¹² Torr, or most preferably in the rangeof about 1×10⁻¹⁰ Torr to about 1×10⁻² Torr may be used in addition tothe particular combinations of features disclosed herein to improvedevice lifetime.

Additional features, advantages, and embodiments of the invention may beset forth or apparent from consideration of the following detaileddescription, drawings, and claims. Moreover, it is to be understood thatboth the foregoing summary of the invention and the following detaileddescription are exemplary and intended to provide further explanationwithout limiting the scope of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, are incorporated in and constitute apart of this specification; illustrate embodiments of the invention andtogether with the detailed description serve to explain the principlesof the invention. No attempt is made to show structural details of theinvention in more detail than may be necessary for a fundamentalunderstanding of the invention and various ways in which it may bepracticed.

FIG. 1 shows an organic light emitting device having separate electrontransport, hole transport, and emissive layers, as well as other layers.

FIG. 2 shows an inverted organic light emitting device that does nothave a separate electron transport layer.

FIG. 3 shows a plot of the normalized luminance versus time and a plotof the current density versus voltage for device in panel I.

FIG. 4 shows a plot of the luminous efficiency versus luminance and aplot of the EL normalized versus wavelength for the device in Panel I.

FIG. 5 shows a plot luminous efficiency versus luminance and a plot ofthe EL normalized versus wavelength for the device in Panel I.

FIG. 6 shows a plot of the percentage power dissipated versus normalizedin-plane wavevector for the device in Panel I.

FIG. 7 shows a plot of the percentage power dissipated versus normalizedin-plane wavevector for the devices in Panel I and Panel II.

FIG. 8 shows a plot of the external quantum efficiency versus luminance,a plot of the power efficacy versus luminance, a plot of luminanceversus voltage and a plot of EL intensity versus wavelength for thedevice in Panel II.

FIG. 9 shows a plot of external quantum efficiency versus luminance, aplot of power efficacy versus luminance, a plot of luminance versusvoltage and a plot of EL intensity versus wavelength for the devicesillustrated in Panels I and II. In the plots, the solid squarescorrespond to the device of Panel I, having a 60 nm EML, and the opensquares correspond to the device of Panel II, having a 30 nm EML.

FIG. 10 shows plots of luminance versus time for the devices illustratedin FIG. 9.

FIG. 11 shows a plot of luminance versus time for the device illustratedin Panel I.

FIG. 12 provides a schematic of the model geometry and assumed energylevel relationships. The recombination zone decays exponentially fromthe EML/ETL interface at x₁ with characteristic length, d_(rec). Bothphosphorescent guests and defects form deep traps within the hostband-gap. The electron and hole quasi-Fermi levels under forward-biasare E_(Fn) and E_(Fv), respectively.

FIGS. 13A-13B: FIG. 13A shows the J-V characteristics of the devicesstudied. Inset: schematic of the device structure, with the dimensions,x₁-x₃ of FIG. 12, indicated as shown. FIG. 13B shows the externalquantum efficiency (left scale) and emission spectrum (right scale)obtained at J=10 mA/cm².

FIGS. 14A-14C: FIG. 14A shows the luminance degradation versus time forinitial brightnesses of L₀=1000, 2000, 3000, and 4000 cd/m² as indicatedby the arrow. Solid black lines indicate a fit using the excitonlocalization degradation model discussed in the text. Note that therecombination zone width, d_(rec), is variable in these fits. The dataare reproduced for comparison with the exciton-exciton degradation modelin FIG. 14B and with the exciton-polaron model in FIG. 14C. All fittingparameters are given in Table 1.

FIGS. 15A-15C: FIG. 15A shows the voltage rise for each of the fourdevices studied. The black lines are calculated using the excitonlocalization model. The data are reproduced in FIG. 15B and FIG. 15C forcomparison with fits from the exciton-exciton and exciton-polaron modelsrespectively. All fitting parameters are given in Table 1.

FIGS. 16A-16C: FIG. 16A shows the photoluminescence transients obtainedfor an as-grown device, one aged to L(t′)=0.59 L_(o)(L₀=1000 cd/m²), andanother aged to L(t′)=0.16 L_(o) of its initial L₀=3000 cd/m²brightness. Solid black lines are fits from the exciton localizationmodel. Predictions of the exciton-exciton annihilation model are shownin FIG. 16B and those for the exciton-polaron annihilation model in FIG.16C. Fitting parameters are given in Table 1.

FIGS. 17A-17C are configurational diagrams showing the differentdissociation mechanisms in terms of energy (E) and representativecoordinate (r). In FIG. 17A, a direct or pre-dissociative potential, R,crosses the singlet or triplet first excited state energy surface. FIG.17B shows the exciton-exciton annihilation process, which leads to aground (S₀) and upper excited state (S_(n)* or T_(n)*) according to thereaction S₁(T₁)+S₁(T₁)→S₀+S_(n)*(T_(n)*). Direct or pre-dissociation mayoccur from the upper excited state (gray arrow, route 1), or it mayrelax vibronically and undergo hot-molecule dissociation (gray arrow,route 2) as discussed in the specification. FIG. 17C shows theexciton-polaron mechanism, in which energy transfer from the excitonresults in an excited polaron that dissociates along the analogousdirect/pre-dissociative and hot-molecule routes. Dotted lines indicatevibrational energy levels within each anharmonic electronic manifold.

FIGS. 18A-18B: FIG. 18A shows the average defect density, Q_(AVG)(t′)and FIG. 18B shows the average defect formation rate, F_(X)(t′), perexciton, per hour, as defined in the text. The curves are calculatedusing the exciton-polaron model, at initial luminances of L₀=1000, 2000,3000, and 4000 cd/m² as indicated by the arrow.

FIGS. 19A-19B: FIG. 19A shows the predicted lifetime improvement atL₀=1000 cd/m², obtained by increasing the recombination zone width,d_(rec). FIG. 19B shows the increase in lifetime calculated for areduction in degradation coefficient, K_(X). Both FIGS. 19A and 1911assume the exciton-polaron model; the filled circles indicate where thedevices of this study lie on the curves.

FIG. 20 shows a plot of the intensity versus wavelength for the devicedescribed in Specific Example 2.

FIG. 21 shows a plot of the partial pressure of gasses in a high vacuumsystem versus the atomic mass unit (amu).

DETAILED DESCRIPTION Definitions

As used herein, the term “organic” includes polymeric materials as wellas small molecule organic materials that may be used to fabricateorganic opto-electronic devices. “Small molecule” refers to any organicmaterial that is not a polymer, and “small molecules” may actually bequite large. Small molecules may include repeat units in somecircumstances. For example, using a long chain alkyl group as asubstituent does not remove a molecule from the “small molecule” class.Small molecules may also be incorporated into polymers, for example as apendent group on a polymer backbone or as a part of the backbone. Smallmolecules may also serve as the core moiety of a dendrimer, whichconsists of a series of chemical shells built on the core moiety. Thecore moiety of a dendrimer may be a fluorescent or phosphorescent smallmolecule emitter. A dendrimer may be a “small molecule,” and it isbelieved that all dendrimers currently used in the field of OLEDs aresmall molecules.

As used herein, “top” means furthest away from the substrate, while“bottom” means closest to the substrate. Where a first layer isdescribed as “disposed over” a second layer, the first layer is disposedfurther away from substrate. There may be other layers between the firstand second layer, unless it is specified that the first layer is “incontact with” the second layer. For example, a cathode may be describedas “disposed over” an anode, even though there are various organiclayers in between.

As used herein, “solution processable” means capable of being dissolved,dispersed, or transported in and/or deposited from a liquid medium,either in solution or suspension form.

A ligand is referred to as “photoactive” when it is believed that theligand contributes to the photoactive properties of an emissivematerial.

The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, arylkyl,heterocyclic group, aryl, aromatic group, and heteroaryl are known tothe art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32,which are incorporated herein by reference.

More details on OLEDs, and the definitions described above, can be foundin U.S. Pat. No. 7,279,704, which is incorporated herein by reference inits entirety.

Generally, an OLED may include at least one organic layer disposedbetween and electrically connected to an anode and a cathode. When acurrent is applied, the anode may inject holes and the cathode injectselectrons into the organic layer(s). The injected holes and electronseach migrate toward the oppositely charged electrode. When an electronand hole localize on the same molecule, an “exciton,” which is alocalized electron-hole pair having an excited energy state, may beformed. Light may be emitted when the exciton relaxes via aphotoemissive mechanism. In some cases, the exciton may be localized onan excimer or an exciplex. Non-radiative mechanisms, such as thermalrelaxation, may also occur, but are generally considered undesirable.

Initially, OLEDs employed emissive molecules that emitted light fromtheir singlet states (“fluorescence”). See, e.g., U.S. Pat. No.4,769,292, which is incorporated by reference in its entirety.Fluorescent emission generally occurs in a time frame of less than about10 nanoseconds. More recently, however, OLEDs having emissive materialsthat emit light from triplet states (“phosphorescence”) have beendemonstrated. See Baldo ET AL., “Highly Efficient PhosphorescentEmission from Organic Electroluminescent Devices,” NATURE, vol. 395,151-154, 1998; (“Baldo-I”) and Baldo ET AL., “Very high-efficiency greenorganic light-emitting devices based on electrophosphorescence,” APPL.PHYS. LETT., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which areincorporated by reference in their entireties. Phosphorescence isdescribed in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, whichare incorporated by reference.

FIG. 1, which illustrates an embodiment, is a schematic showing anorganic light emitting device 100. Device 100 may include a substrate110, an anode 115, a hole injection layer 120, a hole transport layer125, an electron blocking layer 130, an emissive layer 135, a holeblocking layer 140, an electron transport layer 145, an electroninjection layer 150, a protective layer 155, and a cathode 160. Cathode160 may be a compound cathode having a first conductive layer 162 and asecond conductive layer 164. Device 100 may be fabricated by depositingthe layers described, in order. The properties and functions of thesevarious layers, as well as example materials, are described in moredetail in U.S. Pat. No. 7,279,704 at cols. 6-10, which are incorporatedby reference. The terms “anode” and “cathode” are intended to be used intheir broadest sense, to include any layer that injects charge into anOLED. For example, an organic charge generation layer in a stacked OLEDthat injects electrons is considered a “cathode.”

More examples for each of these layers are available. For example, aflexible and transparent substrate-anode combination is disclosed inU.S. Pat. No. 5,844,363, which is incorporated by reference in itsentirety. An example of a p-doped hole transport layer is nm-MTDATAdoped with F₄TCNQ at a molar ratio of about 50:1, as disclosed in U.S.Patent Application Publication No. 2003/0230980, which is incorporatedby reference in its entirety. Examples of emissive and host materialsare disclosed in U.S. Pat. No. 6,303,238, which is incorporated byreference in its entirety. An example of an n-doped electron transportlayer is BPhen doped with Li at a molar ratio of about 1:1, as disclosedin U.S. Patent Application Publication No. 2003/0230980, which isincorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and5,707,745, which are incorporated by reference in their entireties,disclose examples of cathodes including compound cathodes having a thinlayer of metal such as Mg:Ag with an overlying transparent,electrically-conductive, sputter-deposited ITO layer. The theory and useof blocking layers is described in more detail in U.S. Pat. No.6,097,147 and U.S. Patent Application Publication No. 2003/0230980,which are incorporated by reference in their entireties. Examples ofinjection layers are provided in U.S. Patent Application Publication No.2004/0174116, which is incorporated by reference in its entirety. Adescription of protective layers may be found in U.S. Patent ApplicationPublication No. 2004/0174116, which is incorporated by reference in itsentirety.

FIG. 2, which illustrates another embodiment, shows an inverted OLED200. The device may include a substrate 210, a cathode 215, an emissivelayer 220, a hole transport layer 225, and an anode 230. Device 200 maybe fabricated by depositing the layers described, in order. Since themost common OLED configuration has a cathode disposed over the anode,and device 200 has cathode 215 disposed under anode 230, device 200 maybe referred to as an “inverted” OLED. Materials similar to thosedescribed with respect to device 100 may be used in the correspondinglayers of device 200. FIG. 2 provides one example of how some layers maybe omitted from the structure of device 100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided byway of non-limiting example, and it is understood that embodiments ofthe invention may be used in connection with a wide variety of otherstructures. The specific materials and structures described areexemplary in nature, and other materials and structures may be used.Functional OLEDs may be achieved by combining the various layersdescribed in different ways, or layers may be omitted entirely, based ondesign, performance, and cost factors. Other layers not specificallydescribed may also be included. Materials other than those specificallydescribed may be used. Although many of the examples provided hereindescribe various layers as comprising a single material, it isunderstood that combinations of materials, such as a mixture of host anddopant, or more generally a mixture, may be used. Also, the layers mayhave various sublayers. The names given to the various layers herein arenot intended to be strictly limiting. For example, in device 200, holetransport layer 225 transports holes and injects holes into emissivelayer 220, and may be described as a hole transport layer or a holeinjection layer. In one embodiment, an OLED may be described as havingan “organic layer” disposed between a cathode and an anode. This organiclayer may comprise a single layer, or may further comprise multiplelayers of different organic materials as described, for example, withrespect to FIGS. 1 and 2.

Attaining long blue device lifetimes with phosphorescent emitters hasbeen a long-standing problem in OLED device research. Suitable solutionsto this problem would be useful for efficient, long-lived displaydevices as well as new white lighting applications. In order to make acommercially viable fill color OLED display, the industry desires adevice which emits “saturated” blue, i.e., light that has 1931 CIEcoordinates x<0.16, y<0.20 and a device lifetime of at least 10,000hours with an initial luminance of 1000 nits. Notwithstanding manyclaims to improved lifetime materials and devices, device half-livesgreater than about 2000 h starting from an initial luminance of about500 nits have not been previously reported for phosphosphorescent bluedevices with 1931 CIE color coordinates of x<0.17, y<0.27. Accordingly,a novel combination of materials and device architectures are provided,resulting in device half-lives greater than about 10,000 h at 1931 CIEcoordinates x<0.16, 0.26. Some key features of the device, which may beused individually or in various combinations as described herein, mayinclude (i) a thick emissive layer (EML) in direct contact with a holeinjection layer (HIL) in direct contact with the anode, and (ii) asemi-transparent metal anode or cathode, as well as high emissive metalcomplex doping levels, the use of certain families of emitters,including cyclometallated iridium complexes of imidazophenanthridineligands, optimized HOMO and LUMO energy levels, and specific holeinjection, hole transport, host, blocking layer and electron transportmaterials.

It has been found that a model based on exciton-polaron interaction as asource of degraded molecules and ultimately device failure accuratelymatches many of the experimental results reported herein. Ways to reducethis exciton-polaron interaction and increase device lifetime are alsoreported.

According to one embodiment, the long lifetimes of thick EML devicesreflect lower rates of degradation-related exciton-polaron reactions,and also lower rates of exciton quenching by the impurities that may beformed. In particular, at a given luminance, the concentration ofexcitons, polarons and quenchers may be lower in thicker EML devices,thereby making degradative encounters and energy transfers to quenchersless likely and luminance-based lifetimes longer. The term “thick EML,”as used herein generally refers to emissive layers with thicknessesgreater than about 40 nm, greater than about 50 nm, specifically greaterthan about 60 nm, even more specifically greater than about 70 nm, ordevices wherein the EML thickness is greater than the sum of therecombination zone thickness and about 2 times the average excitondiffusion length. The term “recombination zone thickness,” as usedherein generally refers to the thickness of that portion of the EMLwherein about 95% of the recombination occurs. The EML may have athickness as great as about 100 nm or thicker. However, thicknesses inexcess of greater than 100 nm may have only marginal additional benefitsand may undesirably increase fabrication times.

To the extent that excitons reside on the emissive dopant and thatquenchers may also be derived from those same dopants, highconcentrations of the emissive dopant would be expected to increase theaverage exciton-quencher distance, thereby reducing the probability ofenergy transfer, in turn reducing the sensitivity of the device to theaccumulation of quenching impurities. Therefore, some embodiments aredirected to thick EML devices with doping concentrations greater thanabout 9%, specifically greater than about 12%, even more specificallygreater than about 14% to achieve long blue device lifetimes.

For example, if a blue emitter is incorporated in a device where ittransports charge and a significant amount of recombination occurs onthe emitter, then the concentration of the emitter in the host materialmay be greater than about 9 wt. % to enable long lived devices. Thisemitter concentration may be necessary to reduce the operating voltageby allowing more facile transport of holes, to increase the diffusionlength of emitter excitons, to improve the morphological stability ofthe host material, and to disperse decomposition products over a largevolume. The enhancement in lifetime from an initial luminance of about1,000 cd/m² is shown in FIG. 3 where the rate of luminance depreciationwith time, of the OLED shown in FIG. 3, decreases with increasingemitter concentration.

Emitter materials may be stable to holes as inferred from devices whereemitters are used as hole-transport layers or hole-injection layers, anddevice operating voltage decreases with emitter concentration due to itsability to transport holes. As more emitters are added to a host matrix,a network of emitters may be formed where every emitter has anotheremitter as one of its nearest neighbors, so the hopping of holes betweenemitters becomes more efficient hence hole conductivity of the layerimproves and there is corresponding reduction in operating voltage. Alow operating voltage suggests a reduction in internal heating effectsthat may be present in OLED layers.

For example, the operating voltage at about 10 mA/cm² of the device inFIG. 3 was about 7.5 V, about 8.4 V and about 8.8 V for emitterconcentrations of about 15 wt. %, about 12 wt. % and about 9 wt. %.There was about a 14.7% reduction in the power dissipated across thedevice. Specifically, the device should have operating voltages lessthan about 4 V.

If one emitter has a nearest neighbor that is a second emitter, theefficiency of an exciton localized on an emitter to diffuse to thesecond emitter is higher than the efficiency of an exciton moving fromone emitter, that is surrounded by a host material having a tripletenergy greater than the emitter exciton triplet energy (or an energybarrier), to a second emitter. Hence, increasing the emitterconcentration may increase the number of emitters that have emittermolecules as their nearest neighbor and it increases the diffusionlength of emitter excitons.

Emitters generally have higher morphological (thermal) stability thantheir hosts. For example, Ir(ppy)₃ is known to prevent thatcrystallization of CBP. The glass transition temperature is a measure ofthe morphological stability and this temperature may be assumed to be aweighted average of the glass transition temperatures of components of amixed film. Therefore, increasing the concentration of the emitter inthe host may be a way to enhance the morphological stability of films.For example, emitter A in FIG. 3 does not have a glass transitiontemperature; whereas, mCBP and mCP have glass transition temperatures ofabout 95° C. and about 65° C., respectively. The melting points ofemitter A, mCBP and mCP are about 435° C., about 270° C. and about 180°C., respectively.

Degraded molecules may act as exciton quenchers and their concentrationmay directly affect device luminance degradation based on themathematical model of device luminance degradation.

Additionally, the source material used to fabricate an OLED may have apurity that is greater than about 99.5% as determined from high pressureliquid chromatography. This level of purity may be accomplished byperforming at least two re-crystallizations and at least two sublimationpurifications of the source material. Additionally, the concentration ofhalide and metal impurities should be less than about 100 ppm.

Impurities may have many adverse effects on device operational lifetime.Halide, ligand or metal impurities may affect the conductivity of theorganic layers and this could create unfavorable charge balancecharacteristics. The impurities may be chemically reactive and maydestroy emissive molecules to create non-emissive species, or theimpurities may trap charge and act as quenching sites thatsimultaneously contribute to the increase in the operating voltage ofdevices.

For example, FIG. 4 shows the characteristics of a device with emitterB, which had a purity of less than about 99.5%. FIG. 5 shows thecharacteristics of a near identical device as in FIG. 4 but with emitterB having about 99.6% purity and halide impurities less than about 17ppm. The projected device lifetime increased from about 6,000 hrs for alow purity emitter source material to about 10,000 hrs for a high purityemitter source material.

In a further embodiment, optimized HOMO and LUMO levels and selectionsof HIL, HTL, blocking layer and ETL materials and layer thicknesses mayalso be important to achieving optimal charge recombination kineticsinside the emissive layer, thereby avoiding excessive concentration ofrecombination close to the interface of the EML with the other layers.In particular, diffuse charge recombination zones may be employed toreduce the probability of exciton-polaron reactions and increasing theaverage exciton-quencher separation. One way of promoting diffuserecombination and minimizing the likelihood of recombination close to aninterface may be to use grading doping deposition techniques, so thatthere are no sharp interfaces in the device and the dopant concentrationwithin the host is gradually changed. The interfaces in OLED devices maytend to give rise to charge accumulations at the interfaces due tocharge injection barriers that inevitably arise when the chargetransport medium changes, and that when such charge accumulations arewithin diffusion distance of the diffusing excitons, exciton-polaronreactions occur and give rise to quenching impurities. Accordingly,graded doping techniques may be useful in obtaining long devicelifetimes in thick EML blue devices.

The location of the emission region within a device may determine manyaspects of device performance characteristics including its emissionchromaticity, efficiency, efficacy, and operational lifetime. For blueOLED longevity, the emission region may be positioned such that powerdissipation in surface plasmon modes (at the metallic cathode) should beminimized. The dissipation of surface plasmon mode energy is essentiallyheat that can contribute to cathode degradation.

If the dissipation of power (at a wavelength of about 460 nm) is modeledinto surface plasmon modes for two locations within the device structureshown in FIG. 6, more power was dissipated into surface plasmon modeswhen the emission region (modeled as a discrete layer of dipoles) wasnearer to the cathode. Likewise, Device 2 shown in FIG. 7 had lowersurface plasmon power dissipation than Device 1 shown in FIG. 7.Moreover, the recombination zone should be at least about 30 nm awayfrom the metal electrode to reduce the percentage of power in surfaceplasmon modes. (See Hobson ET AL., “Surface Plasmon Mediated Emissionfrom OLEDs” ADV. MATER. 14, 1393 (2002)). A standard software packagefor modeling the optical modes of an OLED can be obtained from FLUXiMAG. Dorfstrasse 7, 8835 Feusisberg, Switzerland. The software mayinclude ETFOS and SETFOS.

Additionally, the unstable nature of emitter anion states may lead tomolecular decomposition of emitters carrying electrons through a device;hence, it is best that the host carries electrons before they recombinewith a hole located on an emitter. One method to clearly determine thatelectrons are transported on the host is shown in FIG. 8. Here, the mCBPEML was divided into doped and undoped regions. Electrons may traversethe undoped mCBP layer to recombine in the doped region, and the resultsshown in FIG. 8 indicate that the efficiency, emission spectra were nearidentical in all cases. Therefore, the recombination location, electrontransport mechanism and optical modes were nearly identical for alldevices, and all the devices in FIG. 8 have identical lifetimes.

Moreover, the cationic state of emitters may be stable to the extentthat emitters can be deposited as neat hole transport or hole injectionlayers and device lifetime can be several thousand hours. Therefore,blue phosphorescent emitters may be made to efficiently trap holes bydecreasing the ionization potential of emitters or raising the HOMOenergy closer to the vacuum level. A deep hole trap may be formed if theHOMO energy of the emitter is less than or equal to about 5.3 eV. Foremitters A and B, their HOMO energies were about 4.8 eV.

Charge (electrons and holes) traverse across organic thin films in OLEDdevices. One standard theory that describes the conduction of charge ishopping, where a charge ‘jumps’ from one molecule to another in adirection determined by the potential difference between the molecules.Molecules may have a stable cationic state (to transport holes) and astable anionic state (to transport electrons) to enable OLEDs with longoperational stabilities. A stable anion or cation is one that does notdecompose (break apart via bond cleavage or other irreparabletransition) during the residence time of a hole or electron on themolecule as the hole or electron ‘jumps’ from molecule to molecule.

The residence time may vary depending on the amount of current flowingthrough the device, and the current may be determined by the mobility,applied potential at the electrodes and concentration of carriers. OLEDsoperate over a large range of current densities in a range of about1×10⁻⁵ mA/cm² to about 10 A/cm², so charge transporting materials areexpected to have stable anionic or cationic states that can support thisrange of current.

To test if a material has a stable anionic or cationic state, acorresponding electron-only or hole-only device may be used to determineif charge flow across the material causes changes in the properties ofthe hole or electron only device. An OLED has a hole and electroncurrent, and the contribution of either current to the total currentvaries across the organic layers.

A device that is described as a hole-only or an electron-only has onlyone charge carrier across all organic layers in the device. Therefore,the cationic stability of a material may be tested if it is incorporatedin a device where only holes traverse the device or the anionicstability of a material can be tested if it is material is incorporatedinto a device where only electrons traverse the device.

If there is any significant change over time of electrical or opticalcharacteristics of a hole only or electron only device that is drivenwith a constant current, the stability of the cationic or anionic statesof the constituent organic materials may be poor. If the operatingvoltage of a hole-only or electron-only device increases by about 0.2 Vor if the PL quantum yield of the organic materials decreases by about5%, the stability of the cationic or anionic state may be poor.

Sharp interfaces may be formed when two distinct materials arejuxtaposed. The nature of the interface formed may have highconcentration of charge due to energetic barriers to the transport ofcharge across the interface. The high concentration of chargeeffectively leads to enhanced molecular degradation and shorter devicelifetimes.

Additionally, the method of fabricating such interfaces may allowimpurities to be deposited and incorporated into devices. For example,vacuum thermal evaporation may be used to fabricate devices. Each layermay be deposited sequentially and there is typically a few minutesbetween the depositions of distinct layers when impurities in the vacuumsystem impinge of the surface of the freshly deposited material. Theseimpurities may lead to degradation, be sites for trapped charge andalter device characteristics.

Ultra high vacuum systems may therefore be used to provideblue-phosphorescent OLEDs with improved operational longevity. Ultrahigh vacuum system have vacuum levels less than about 1×10⁻⁸ Torr,preferably in the range of about 1×10⁻⁸ Torr to about 1×10⁻¹² Torr morepreferably in the range of about 1×10⁻⁹ Torr to about 1×10⁻¹² Torr, ormost preferably in the range of about 1×10⁻¹⁰ Torr to about 1×10⁻¹²Torr. In high vacuum systems, which have higher pressure than ultra highvacuum systems, the major constituent gas is water, as shown in FIG. 21.In contrast to the high vacuum system, the ultra high vacuum systemprovides an environment with minimal moisture and therefore may minimizethe decomposition of materials known to be photo-oxidatively unstable.First, the ultra high vacuum system may enable atomically cleaninterfaces between various organic materials comprising an OLED. Second,the ultra high vacuum system may prevent the co-deposition of water andother impurities with organic materials that are highly reactive andreadily undergo photo-oxidation. Taken together, the ultra high vacuumsystem may be used to provide improved blue devices.

Blue photoemissive materials, such as Compounds A and B and othercompounds of the same family, are known to be highly photo-oxidativelyunstable. In the presence of moisture or oxygen and light, members ofthis family of materials decompose. For example, Compound A may oxidizeto

Moreover, the blue phosphorescent OLEDs containing Compounds A and Bhave traditionally been fabricated in high vacuum systems where moistureis readily incorporated into the organic layers and at the interfaces asexplained below. Therefore, these emitters can readily photo-activelydecompose, because all components required for the reaction (light plussource of oxygen) are present when the devices are turned on and emitlight.

Given that the pressure of the ultra high vacuum environment may enableatomically clean surfaces and prevent deposition of impurities withreactive organic materials, the variation of various parameters withrespect to pressure are considered below. In particular, theseparameters include (i) gas density, (ii) mean free path of particles inthe gas phase, (iii) incident molecular flux on surfaces, (iv) gasexposure, and (v) sticking coefficient and surface coverage.

Gas density. The gas density can be estimated from the ideal gas law,given by the equation:

n=(N/V)=P/(kT)[molecules m⁻³],

where P is the pressure [N m⁻²], k is Boltzmann constant and T istemperature in Kelvin.

Mean Free Path of Particles in the Gas Phase. The average distance thata particle (e.g., atom, electron, molecule) travels in the gas phasebetween collisions can be determined from a simple hard-sphere collisionmodel (see, for example, Atkins' Physical Chemistry). This quantity isknown as the mean free path of the particle, here denoted by λ[m], andfor neutral molecules is given by the equation:

${\lambda = \frac{kT}{1.414P\; \sigma}},$

where σ is the collision cross section [m²].

Incident Molecular Flux on Surfaces. The number of gas moleculesimpacting on the surface from the gas phase is a crucial factor indetermining how long a surface can be maintained clean or,alternatively, how long it takes to build-up a certain surfaceconcentration of adsorbed species. The incident flux, FP is defined asthe number of incident molecules per unit time per unit area of surface.Of note, the flux takes no account of the angle of incidence, instead itis merely a summation of all the arriving molecules over all possibleincident angles.

For a given set of conditions (e.g., P, T, etc.), the flux can bereadily calculated using a combination of the ideas of (i) statisticalphysics, (ii) the ideal gas equation, and (iii) the Maxwell-Boltzmanngas velocity distribution. (i) The incident flux, F [molecules m⁻² s⁻¹],can be shown to be related to the gas density above the surface by theequation F=¼ n c, where n is the molecular gas density [molecules m⁻³]and c is the average molecular speed [ms⁻¹]. (ii) The molecular gasdensity can be given by the ideal gas equation, namely n=(N/V)=P/(kT)[molecules m⁻³]. (iii) The mean molecular speed can be obtained from theMaxwell-Boltzmann distribution of gas velocities by integration,yielding

$\overset{\_}{c} = {\sqrt{\frac{8{kT}}{m\; \pi}}.}$

Combining the equations provided above gives the Hertz-Knudsen formulafor the incident flux,

$F = {\frac{P}{\sqrt{2\pi \; {mkT}}}.}$

Gas Exposure. The gas exposure can be defined as a measure of the amountof gas to which a surface has been subjected. It can be numericallyquantified by taking the product of the pressure of the gas above thesurface and the time of exposure if the pressure is constant, or moregenerally by calculating the integral of pressure over the period oftime of concern. Although the gas exposure may be given in the SI unitsof Pa s (Pascal seconds), the normal and far more convenient unit forexposure is the Langmuir, where 1 L=10⁻⁶ Torr s, or as provided by theequation

(Exposure/L)=10⁶×(Pressure/Torr)×(Time/s).

Sticking Coefficient and Surface Coverage. The sticking coefficient, S,is defined as a measure of the fraction of incident molecules whichadsorb upon the surface (i.e., a probability and lies in the range 0-1,where the limits correspond to no adsorption and complete adsorption ofall incident molecules respectively. In general, S depends upon manyvariables, such that S=f (surface coverage, temperature, crystal face,etc.)

A monolayer (1 ML) of adsorbate corresponds to the maximum attainablesurface concentration of adsorbed species bound to the substrate. Theamount of time for a clean surface to become covered with a completemonolayer of adsorbate can be dependent upon the flux of gas phasemolecules incident upon the surface, the actual coverage correspondingto the monolayer and the coverage-dependent sticking probability.However, a minimum estimate of the time required may be obtained byassuming a unit sticking probability (i.e., S=1) and noting thatmonolayer coverage is generally on the order of about 10¹⁵ per cm² orabout 10¹⁹ per m², providing the equation Time/ML˜(10¹⁹/F) [s]. Table 1provides a comparison of the time required to deposit a monolayer atvarious levels of vacuum. The time to deposit a monolayer, the gasdensity and the mean free path are listed for various system pressures.All values given below are approximate and are generally dependent onfactors such as temperature and molecular mass.

TABLE 1 Degree of Pressure Gas Density Mean Free Path Time/ML Vacuum(Torr) (molecules m⁻³) (m) (s) Atmospheric 760 2 × 10²⁵ 7 × 10⁻⁸ 10⁻⁹Low  1 3 × 10²² 5 × 10⁻⁵ 10⁻⁶ Medium  10⁻³ 3 × 10¹⁹ 5 × 10⁻² 10⁻³ High 10⁻⁶ 3 × 10¹⁶ 50  1 UltraHigh  10⁻¹⁰ 3 × 10¹² 5 × 10⁵ 10⁴

Graded interfaces may be formed by mixing two or more distinct materialsin a fashion that an abrupt change from one layer to another is notformed. Graded interfaces have been shown to improve device lifetime andthis device architecture may be beneficial to improving PHOLED lifetimebased on the degradation model, where degradation is concentrated nearan abrupt interface.

FIG. 9 shows a plot of external quantum efficiency versus luminance, aplot of power efficacy versus luminance, a plot of luminance versusvoltage and a plot of EL intensity versus wavelength for the devicesillustrated in Panels I and II. In the plots, the solid squarescorrespond to the device of Panel I, having a 60 nm EML, and the opensquares correspond to the device of Panel II, having a 30 nm EML. FIG.10 shows plots of luminance versus time for the devices illustrated inFIG. 9. The device of Panel I shows higher external quantum efficiency,higher power efficacy, higher luminance, and a similar emission spectrawhen compared to the device of Panel II. The device of Panel I alsoshows a longer lifetime than the device of Panel II. The improvedperformance of the device of Panel I can be attributed to a combinationof features: (1) a thicker EML, which may allow excitons to diffuse andtherefore have a lower concentration at any given position in the EML;(2) a greater distance between the cathode and the EML (or the emissiveregion), which may result in reduced surface plasmon mode.

FIG. 11 shows a plot of luminance versus time for the devicesillustrated in Panel I. The only difference between the devices is thatone uses dopant A (illustrated in FIG. 3) and the other uses dopant B(illustrated in FIG. 4). Dopant A is basically dopant B with a neopentylgroup added. The device with dopant A shows a longer LT50 lifetime, andalso has a lower rate of degradation at 50% of the initial luminance.The superior performance of the device with dopant A is attributed tothe presence of the neopentyl substituent.

Another embodiment relates to minimizing the likelihood that an excitonwill diffuse to encounter a polaron or a quencher by designing dopants,hosts and co-hosts that minimize the exciton diffusion length. Within agiven family of dopants, this may be achieved by increasing theperipheral steric bulk of the dopant in such away that dopant to dopantenergy migration is inhibited, relative to an otherwise identical dopantlacking the hindrance. One way to introduce steric hindrance may be tosubstitute the periphery of a cyclometallated imidazophenanthridineligand with neopentyl substituents. In particular, neopentylsubstituents may shield one face of the aromatic ring to which they areattached, and because they still possess sufficient degrees of freedomto increase the entropy of sublimation and thereby reduce thetemperature and increase the cleanliness of sublimation. Therefore,incorporating neopeltyl and related R₃CCH and R₃SiCH₂ substituents,where R is hydrocarbyl or heteroatom-substituted hydrocarbyl, on theperiphery of the ligands which comprise the emissive dopant in thick EMLdevices may be employed.

Since particular metal complexes may be photo-oxidatively sensitive, andmay form green or red-emissive impurities upon undue simultaneousexposure to air and light. According to one embodiment of the invention,phosphorescent metal complexes where the presence of residual impuritiesarising from photo-oxidation has been minimized such that the ratio ofthe emission intensity at the pure emitter peak emission wavelength tothe intensity at the peak wavelength plus about 80 nm is at least about100:1 may be used.

The photoluminescence and electroluminescence of an organic material maybe altered by exposure to various gases. These materials (emitters) maybe doped into an organic host matrix and the photo- andelectroluminescence color of the doped film may be controlled bycontrolling the amount of time and quantity of gas exposed to the dopedfilm, since some dopants may change color while others may not changecolor. Therefore, the emission color of the film from its initial colormay be precisely tuned, or to an intermediate color where some dopantshave changed color, or to the color where all dopant molecules havechanged color. The ability to change the color of a dopant afterdeposition of the material may have several unique applications for thefabrication of organic light emitting devices (OLEDs). For example,white OLEDs may be fabricated by carefully controlling the exposure ofan initially blue emissive doped film to a reactive element that makesthe dopant emit green or red such that some dopants continue to emitblue and some emit red and/or green to create white emission. Since theexposure to a gas can cause the emission color of the dopant to change,devices may be created that can be used as sensors for the particulargas.

For monochromatic emitters, the photo-oxidation process is highlyundesirable and may be avoided by minimizing exposure to light duringthe synthesis of materials and fabrication of devices, and by ensuringthat the material is not exposed to more than 10 ppm of oxygen. It isbelieved that light having a wavelength <600 nm may be responsible formuch of the undesirable photo-oxidation, so avoiding exposure to suchlight during fabrication is desired. Avoiding exposure to any lightduring fabrication may also be desirable. The “near absence” of lightmeans less than 0.1 1× as measured on an illuminance meter, such as theMinolta T-10.

Additionally, the photo-oxidation process may be avoided by minimizingexposure to moisture during the synthesis of materials and devices. Itis believed that the presence of moisture may be responsible fordecomposition of photoemissive compounds, so avoiding exposure tomoisture during fabrication is desired. An ultra high vacuum systemprovides a cleaner environment, including a reduction in the moisturepresent in the environment. Therefore, organic materials can bedeposited in a vacuum system having vacuum levels less than about 1×10⁻⁸Torr, preferably in the range of about 1×10⁻⁸ Torr to about 1×10⁻¹²Torr, more preferably in the range of about 1×10⁻⁹ Torr to about 1×10⁻¹²Torr, or most preferably in the range of about 1×10⁻¹⁰ Torr to about1×10⁻¹² Torr. Avoiding exposure to light, exposure to oxygen andexposure to moisture during device fabrication may be used incombination with other measures to obtain high purity eitherindividually or, preferably, in combination.

Emitters should be capable of being sublimed from a heat source underhigh vacuum conditions. The sublimation temperature of the emittershould be less than about 350° C. when the distance from the heatedsource to the OLED substrate exceeds about 50 cm and the film thicknessis formed at less than about 0.3 nm/s. Decomposition of the sourcematerial during sublimation is undesirable. Decomposed source materialtypically forms residue, which cannot be sublimed below about 350° C.The residue may consist of fragments of the original molecule orimpurities such as halides and other materials used in the synthesis ofthe source material.

In yet a further embodiment, devices are provided that may be fabricatedby sublimation of the emissive dopant. In such cases, in order toachieve long device lifetimes, it is important that the dopant sublimecleanly. Therefore, the dopant may be designed to have sufficientstability towards sublimation as to give an HPLC assay of at least about98% and leave a residue corresponding to less than about 5 wt % of theoriginal charge in the sublimation crucible. Less than 5 wt % remainsafter the original charge is fully discharged, i.e., where there is nomore appreciable deposition form the crucible when the depositionapparatus is operated under normal conditions. This criteria does notrequire that the crucible is fully discharged when it is used to makedevices. Rather, the criteria is that the source material has sufficientstability under normal deposition conditions, that, if the crucible isfully discharged the residue is less than about 5 wt % of the originalcharge.

In further embodiments, structures and materials not specificallydescribed herein may also be used, such as OLEDs comprised of polymericmaterials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190, which isincorporated by reference in its entirety. By way of further example,OLEDs having a single organic layer may be used. OLEDs may be stacked,for example as described in U.S. Pat. No. 5,707,745, which isincorporated by reference in its entirety. The OLED structure maydeviate from the simple layered structure illustrated in FIGS. 1 and 2.For example, the substrate may include an angled reflective surface toimprove out-coupling, such as a mesa structure as described in U.S. Pat.No. 6,091,195, and/or a pit structure as described in U.S. Pat. No.5,834,893, which are incorporated by reference in their entireties.

Unless otherwise specified, any of the layers of the various embodimentsmay be deposited by any suitable method. For the organic layers,particular methods include thermal evaporation, ink-jet, such asdescribed in U.S. Pat. Nos. 6,013,982 and 6,087,196, which areincorporated by reference in their entireties, organic vapor phasedeposition (OVPD), such as described in U.S. Pat. No. 6,337,102, whichis incorporated by reference in its entirety, and deposition by organicvapor jet printing (OVJP), such as described in U.S. patent applicationSer. No. 10/233,470, which is incorporated by reference in its entirety.Other suitable deposition methods include spin coating and othersolution based processes. Solution based processes may be carried out innitrogen or an inert atmosphere. For the other layers, particularmethods include thermal evaporation. Specific patterning methods includedeposition through a mask, cold welding such as described in U.S. Pat.Nos. 6,294,398 and 6,468,819, which are incorporated by reference intheir entireties, and patterning associated with some of the depositionmethods such as ink-jet and OVJD. Other methods, however, may also beused.

The materials to be deposited may be modified to make them compatiblewith a particular deposition method. For example, substituents such asalkyl and aryl groups, branched or unbranched, and preferably containingat least about 3 carbons, may be used in small molecules to enhancetheir ability to undergo solution processing. Substituents having about20 carbons or more may be used, and specifically in a range of about 3to about 20 carbons may be used. Materials with asymmetric structuresmay have better solution processability than those having symmetricstructures, because asymmetric materials may have a lower tendency torecrystallize. Dendrimer substituents may be used to enhance the abilityof small molecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the invention maybe incorporated into a wide variety of consumer products, including flatpanel displays, computer monitors, televisions, billboards, lights forinterior or exterior illumination and/or signaling, heads up displays,fully transparent displays, flexible displays, laser printers,telephones, cell phones, personal digital assistants (PDAs), laptopcomputers, digital cameras, camcorders, viewfinders, micro-displays,vehicles, a large area wall, theater or stadium screen, or a sign.Various control mechanisms may be used to control devices fabricated inaccordance with the present invention, including passive matrix andactive matrix. Many of the devices are intended for use in a temperaturerange comfortable to humans, such as a temperature in a range of about18° C. to about 30° C., and more particularly at room temperature (about20° C. to about 25° C.).

The materials and structures described herein may have applications indevices other than OLEDs. For example, other optoelectronic devices suchas organic solar cells and organic photodetectors may employ thematerials and structures. More generally, organic devices, such asorganic transistors, may employ the materials and structures.

Based on the exciton-polaron model and as demonstrated by the modelimmediately below, it is believed that certain features and/orcombinations of features result in blue devices having surprisingly longlives. The following list of features and combinations of features areexemplary and are not intended to be exhaustive.

In one aspect, charge carriers and solid state considerations arefeatures that may result in a device having a longer lifetime. A devicemay have an emissive layer that includes a host having a triplet energyand a phosphorescent emissive dopant having a peak emissive wavelengthless than 500 nm. The device also includes an exciton blocking layer atleast 5 nm thick disposed between the emissive layer and the cathode andadjacent to the emissive layer, the exciton blocking layer consistingessentially of materials having a triplet energy greater than or equalto the triplet energy of the host of the emissive layer. The emissivelayer is at least 40 nm thick. Electrons in the emissive layer arecarried predominantly by the host. The HOMO of the phosphorescentemissive dopant is at least 0.5 eV higher than the HOMO of the host. Theconcentration of the phosphorescent emissive dopant in the emissivelayer is at least 9 wt %. The phosphorescent emissive dopant comprises acyclometallated N,C-donor imidazophenanthridine ligand comprising atleast one 2,2,2-trialklethyl substituent. It is believed that thiscombination of features leads to a device having an unexpectedly longlifetime.

Preferably, this specific set of features described in the precedingparagraph is used in combination with an ultra high vacuum system. Inparticular, an ultra high vacuum system having a pressure level lessthan about 1×10⁻⁸ Torr, preferably in the range of about 1×10⁻⁸ Torr toabout 1×10⁻¹² Torr, more preferably in the range of about 1×10⁻⁹ Torr toabout 1×10⁻¹² Torr, or most preferably in the range of about 1×10⁻¹⁰Torr to about 1×10⁻¹² Torr may be used in combination with the abovefeatures to improve device lifetime.

In one aspect, emitter purity is a feature that may result in deviceshaving longer lifetime. In particular, a device that includes, interalia, an emissive layer including a host and a phosphorescent emissivedopant having a peak emissive wavelength less than 500 nm and where (i)the phosphorescent emissive dopant is deposited from a source that has apurity in excess of about 99.5% as determined by high pressure liquidchromatography, (ii) the source further having a combined concentrationof halide and metal impurities below about 100 ppm, (iii) thephosphorescent emissive dopant leaves a residue corresponding to lessthan about 5 wt % of the original charge in the sublimation crucible,and (iv) the phosphorescent emissive dopant has a sublimationtemperature less than about 350° C. and is deposited via sublimation mayhave a surprisingly long lifetime.

Preferably, this specific set of features described in the precedingparagraph is used in combination with an ultra high vacuum system. Inparticular, an ultra high vacuum system having a pressure level lessthan about 1×10⁻⁸ Torr, preferably in the range of about 1×10⁻⁸ Torr toabout 1×10⁻¹² Torr, more preferably in the range of about 1×10⁻⁹ Torr toabout 1×10⁻¹² Torr, or most preferably in the range of about 1×10⁻¹⁰Torr to about 1×10⁻¹² Torr may be used in combination with the abovefeatures to improve device lifetime.

In one aspect, charge carriers and solid state considerations arefeatures that may result in a device having a longer lifetime. Inparticular, a device including an emissive layer disposed between theanode and the cathode, such that the emissive layer includes a host anda phosphorescent emissive dopant having a peak emissive wavelength lessthan about 500 nm and such that when a voltage is applied across thedevice, electrons in the emissive layer are carried predominantly by thehost and where the (i) HOMO of the phosphorescent dopant is at leastabout 0.5 eV higher than the HOMO of the host and (ii) thephosphorescent emissive dopant has a concentration of at least about 9wt % in the emissive layer may have a surprisingly longer lifetime.

Preferably, this specific set of features described in the precedingparagraph is used in combination with an ultra high vacuum system. Inparticular, an ultra high vacuum system having a pressure level lessthan about 1×10⁻⁸ Torr, preferably in the range of about 1×10⁻⁸ Torr toabout 1×10⁻¹² Torr, more preferably in the range of about 1×10⁻⁹ Torr toabout 1×10⁻¹² Torr, or most preferably in the range of about 1×10⁻¹⁰Torr to about 1×10⁻¹² Torr may be used in combination with the abovefeatures to improve device lifetime.

In one aspect, structural and optical considerations are features thatmay result in a device having a longer lifetime. For example, a deviceincluding a CuPc hole injection layer disposed between the anode and thecathode and adjacent to the anode, an emissive layer disposed betweenthe anode and the cathode such that the emissive layer includes a hostand a phosphorescent emissive dopant having a peak emissive wavelengthless than about 500 nm, and an exciton blocking at least about 5 nmthick disposed between the emissive layer and the cathode and adjacentto the emissive layer such that the exciton blocking layer consistingessentially of materials having a triplet energy greater than or equalto the host of the emissive layer and where (i) the emissive layer is atleast about 40 nm thick, and (ii) the emissive layer is at least about40 nm from the cathode may have a surprisingly longer lifetime.

Preferably, this specific set of features described in the precedingparagraph is used in combination with an ultra high vacuum system. Inparticular, an ultra high vacuum system having a pressure level lessthan about 1×10⁻⁸ Torr, preferably in the range of about 1×10⁻⁸ Torr toabout 1×10⁻¹² Torr, more preferably in the range of about 1×10⁻⁹ Torr toabout 1×10⁻¹² Torr, or most preferably in the range of about 1×10⁻¹⁰Torr to about 1×10⁻¹² Torr may be used in combination with the abovefeatures to improve device lifetime.

In one aspect, the device may include a CuPc hole injection layerdisposed between the anode and the cathode and adjacent to the anode, anemissive layer disposed between the anode and the cathode such that theemissive layer includes a host and a phosphorescent emissive dopanthaving a peak emissive wavelength less than about 500 nm, and an excitonblocking at least about 5 nm thick disposed between the emissive layerand the cathode and adjacent to the emissive layer such that the excitonblocking layer consisting essentially of materials having a tripletenergy greater than or equal to the host of the emissive layer and where(i) the emissive layer is at least 40 nm thick; and (ii) the surfaceplasmon mode less than about 30% may have a longer lifetime.

In one aspect, a device is provided having an anode, a cathode, and anemissive layer disposed between the anode and the cathode. The emissivelayer includes an organic phosphorescent emissive dopant, and an organiccarbazole host material. The organic carbazole host material has thestructure:

where R₁ and R₂ denote possible substitution at any available carbonatom or atoms of the indicated ring by alkyl or aryl groups, which maybe further substituted. R₃ is an alkyl or aryl group, which may befurther substituted. Preferably, the organic carbazole host material hasthe structure:

where R is an aryl group, which may be further substituted. Preferably,the carbazole host has a triplet energy corresponding to a wavelengthless than 480 nm. More preferably, the carbazole host is mCBP, havingthe structure:

Preferably, the organic emissive dopant has a peak emissive wavelengthless than 500 nm. Preferably, the organic phosphorescent emissive dopantis organometallic. More preferably, the organic phosphorescent dopant isan organometallic material having at least one organic ligandcoordinated to an Ir atom. More preferably, the organic phosphorescentdopant has the structure A:

Devices with this desirable combination of host and dopant in theemissive layer may have unexpectedly long lifetimes. The devicesillustrated in FIGS. 3 and 8 are examples of the combination of host anddopant described in this paragraph. Any combination of host, fromgeneral to specific, and dopant, from general to specific, described inthis paragraph is desirable.

Preferably, this specific set of features described in the precedingparagraph is used in combination with an ultra high vacuum system. Inparticular, an ultra high vacuum system having a pressure level lessthan about 1×10⁻⁸ Torr, preferably in the range of about 1×10⁻⁸ Torr toabout 1×10⁻¹² Torr, more preferably in the range of about 1×10⁻⁹ Torr toabout 1×10⁻¹² Torr, or most preferably in the range of about 1×10⁻¹⁰Torr to about 1×10⁻¹² Torr may be used in combination with the abovefeatures to improve device lifetime.

Experimental and Modeling

According to one embodiment, the fundamental mechanisms leading todegradation during long term operation of a typical, blueelectrophosphorescent OLED are described. The trends inelectrophosphorescence decay, voltage rise, and emissive layerphotoluminescence quenching associated with electrical aging may be bestfit to a model (infra) based on the assumption that defect sitesgenerated during operation act as exciton quenchers, deep charge traps,and nonradiative recombination centers, Defect generation due toexciton-polaron annihilation interactions between the dopant and hostmolecules may lead to model predictions in good agreement with the data.Moreover, a link between guest exciton energy and the annihilationinduced defect formation rate may be suggested, with increasing guestemission energy leading to increased defect formation rates.Accordingly, the model may provide that the operational lifetime of blueOLEDs may be less than green and red due to the higher energyexcitations of the former system. Finally, defect densities of about10¹⁸ cm⁻³ may result in greater than about 50% degradation from initialluminance.

Depending on their energy levels, defects may act as luminescentquenchers, non-radiative recombination centers, and deep charge traps.Luminance loss may result from the first two, while voltage rise, whichhas been linked to the presence of fixed space charge in the emissiveregion, may result from filling of the deep traps. This is depictedschematically in FIG. 12 for a single, discrete, deep defect state atenergy, E_(t), that lies between the highest occupied (HOMO) and lowestunoccupied molecular orbitals (LUMO) of the host. The energetics of thephosphorescent guest are also shown in FIG. 12. Thus, both defect andguest form discrete, deep hole traps that allow for direct excitonformation, although recombination is only radiative on the guest.Luminescence quenching by defects may occur if there exists an allowedtransition resonant with that of the guest or host that enables Forsteror Dexter energy transfer to occur.

Defects may be assumed to act only as hole traps, with E_(t)representing the defect HOMO. In general, however, defects may have boththeir HOMO and LUMO, or a singly occupied molecular orbital within thehost band gap, creating both electron and hole traps. In addition, thedefect state itself may not lead directly to a quenching transition, butwhen occupied with a trapped charge, the resulting polaron might becomea quenching center. The model presented below is general, however, andalthough it has been derived for the specific case shown in FIG. 12, itnevertheless remains applicable to these alternative scenarios, asdiscussed below.

According to one embodiment, a single recombination zone that may decayexponentially from one edge of the emissive layer (EML) withcharacteristic length d_(rec), is depicted in FIG. 12. High efficiencyelectrophosphorescent OLEDs may have a charge balance factor near unity,hence it is assumed that equal numbers of electrons and holes enter therecombination zone. Excitons formed on the host may then rapidlytransferred and localized on the phosphorescent guests as a result oftheir high doping concentration and host triplet energy. As a result,exciton diffusion out of the recombination zone may be negligible.

These considerations lead to rate equations for hole (p), electron (n),and exciton (N) densities in the recombination zone as follows:

$\begin{matrix}{\frac{{p( {x,t,t^{\prime}} )}}{t} = {{\frac{J}{{qd}_{rec}( {1 - {\exp ( {{- ( {x_{2} - x_{1}} )}/d_{rec}} )}} )}{\exp ( {{- ( {x - x_{1}} )}/d_{rec}} )}} - {\gamma \; {n( {x,t,t^{\prime}} )}{p( {x,t,t^{\prime}} )}} - {\sigma \; {v_{th}\lbrack {f_{D}( E_{t} )} \rbrack}{Q( {x,t^{\prime}} )}{p( {x,t,t^{\prime}} )}}}} & \lbrack 1\rbrack \\{\frac{{n( {x,t,t^{\prime}} )}}{t} = {{\frac{J}{{qd}_{rec}( {1 - {\exp ( {{- ( {x_{2} - x_{1}} )}/d_{rec}} )}} )}\exp ( {{- ( {x - x_{1}} )}/d_{rec}} )} - {\gamma \; n( {x,t,t^{\prime}} ){p( {x,t,t^{\prime}} )}} - {{\gamma_{2}\lbrack {1 - {f_{D}( E_{t} )}} \rbrack}{Q( {x,t^{\prime}} )}{n( {x,t,t^{\prime}} )}}}} & \lbrack 2\rbrack \\{\frac{{N( {x,t,t^{\prime}} )}}{t} = {{\gamma \; {n( {x,t,t^{\prime}} )}{p( {x,t,t^{\prime}} )}} - {( {{1/\tau} - {K_{DR}{Q( {x,t^{\prime}} )}}} ){N( {x,t,t^{\prime}} )}}}} & \lbrack 3\rbrack\end{matrix}$

The electron, hole, and exciton densities may depend on the time scaleof transport and energy level transitions, t (short), as well as on thatof degradation, t′ (long), due to formation of defects of densityQ(x,t′). The electron and hole densities may be functions of the currentdensity, J, the elementary charge q, and the device dimensions shown inFIG. 12. Excitons may be formed at the Langevin rate,γ=q(μ_(n)+μ_(p))/(∈∈₀), and decay with natural lifetime, τ. The hole andelectron mobilities in the doped emissive layer are μ_(p) and μ_(n),respectively, the relative dielectric constant of the emissive layer is∈≈3, and ∈₀ is the permittivity of free space.

In Equation (1), holes with thermal velocity, v_(th)˜10⁷ cm/s, trap atdefect sites of energy, E_(t), and cross section, σ. The Fermi factor,ƒ_(D)(E_(t))=[exp(E_(t)−E_(Fv))+1]⁻¹, gives the probability that thehole trap is empty, where E_(ƒv) is the hole quasi-Fermi energy.Electrons in Equation (2) non-radiatively may recombine at a rateproportional to the trapped hole density, Q(x,t′)[1−ƒ_(D)(E_(t))], andthe reduced Langevin coefficient, γ₂=q(μ_(n))/(∈∈₀), since trapped holesare assumed to be immobile. Quenching of excitons by defects isdescribed by the bimolecular rate coefficient, K_(DR), in Equation (3).Note that only the constant prefactors change if defects trap electrons,or both carrier types, instead of only holes, as considered above.

The defect generation mechanism has four possible routes:

$\frac{{Q( {x,t^{\prime}} )}}{t^{\prime}} = \{ \begin{matrix}{K_{X}{n( {x,t^{\prime}} )}} & {K_{X}{{p( {x,t^{\prime}} )}\lbrack {4a} \rbrack}} \\{K_{X}{{N( {x,t^{\prime}} )}\lbrack {4b} \rbrack}} & \; \\{K_{X}{{N^{2}( {x,t^{\prime}} )}\lbrack {4c} \rbrack}} & \; \\{{K_{X}{N( {x,t^{\prime}} )}{n( {x,t^{\prime}} )}},} & {K_{X}{N( {x,t^{\prime}} )}{{p( {x,t^{\prime}} )}\lbrack {4d} \rbrack}}\end{matrix} $

where the rate constant, K_(X), is consistent in dimension with theorder of the reaction. In Equation (4a), the presence of an electron orhole (i.e. a polaron) leads to molecular degradation, while in Equation(4b) excitons are responsible. Defect formation is a product ofexciton-exciton annihilation in Equation (4c), and of exciton-polaron(hole or electron) annihilation in Equation (4d).

On the short time scale, t, Equations (1)-(3) are at steady state andmay be solved to yield an expression for N (x,t′). The resulting,coupled differential equations containing N(x,t′) and Q(x, t′) may thensolved numerically. Thus, the normalized OLED luminescence as a functionof time is:

$\begin{matrix}{{{{El}_{norm}( t^{\prime} )} = \frac{\int_{x_{1}}^{x_{2}}{N( {x,t^{\prime}} )}}{\int_{x_{1}}^{x_{2}}{N( {x,0} )}}},} & \lbrack 5\rbrack\end{matrix}$

and the defect formation rate per exciton, averaged over therecombination zone is:

$\begin{matrix}{{F_{X}( t^{\prime} )} = {\frac{1}{d_{rec}}{\int_{x_{1}}^{x_{2}}{\frac{1}{N( {x,t^{\prime}} )}\frac{{Q( {x,t^{\prime}} )}}{t^{\prime}}{{x}.}}}}} & \lbrack 6\rbrack\end{matrix}$

Here, the integration limits, x₁ and x₂, are defined in FIG. 12. Thedensity of trapped charge increases with defect density following:ρ_(T)(x,t′)=qQ(x,t′)[1−ƒ_(D)(E_(t))]. Assuming that the growth of ρ_(T)is offset by an equal density of opposing charge at the cathode, andthat the free charge distributions under steady-state operation are notperturbed, then the voltage rise is given by:

ΔV(t′)≈∫₀ ^(x) ³ xρ_(T)(x,t′)dx.  [7]

The emissive layer photoluminescence (PL) transient will also beaffected by defects. From Equation (3) at time, t′, the PL intensitynormalized to that at t=0 is:

$\begin{matrix}{ {{PL}_{norm}(t)} |_{t^{\prime}} = {\frac{\int_{x_{1}}^{x_{2}}{{I_{0}(x)}{\exp \lbrack {{- ( {{1/\tau} + {K_{DR}{Q( {x,t^{\prime}} )}}} )}t} \rbrack}}}{\int_{x_{1}}^{x_{2}}{I_{0}(x)}}.}} & \lbrack 8\rbrack\end{matrix}$

Here, I₀(x) is the intensity profile of the excitation pulse in thedevice emissive layer calculated by the transfer matrix method for thespecific device structure, incident excitation angle, wavelength, andpolarization considered.

EXAMPLES Specific Example 1

Indium-tin-oxide (ITO) coated glass was cleaned with solvents andpatterned into 2 mm² anode contact areas using standard photolithographytechniques prior to organic film deposition. The ITO was oxygen plasmacleaned, exposed to UV-ozone treatment, and then loaded into a vacuumchamber with a base pressure of about 10⁻⁷ Torr. The device structurewas as follows for the device of FIG. 13: a 10 nm thick hole injectionlayer 1310, a 30 nm thick layer of the hole transporting4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (NPD) 1312, a 30 nmthick emissive layer 1314 including mCBP doped with about 9 wt % of theblue phosphorfac-tris[3-(2,6-dimethylphenyl)-7-methylimidazo[1,2-f]phenanthridine]Iridium(III),shown as molecule B in FIG. 4, and a 5 nm thick layer of mCBP 1316 forexciton confinement within the EML. Electrons were injected into the EMLthrough about a 40 nm thick layer of tris-(8-hydroxyquinoline) aluminum1318, capped by a cathode 1320 including of about a 0.8 nm thick layerof LiF 1322 and about a 100 nm thick Al film 1324. Following deposition,the OLEDs were transferred directly from vacuum into an oxygen andmoisture-free N₂ glove box, and were subsequently encapsulated using aUV-curable epoxy, and a glass lid containing a moisture getter.

External quantum (EQE) and power efficiencies were calculated from thespectral intensity measured normal to the substrate using a SpectraScanPR705. The current and voltage measurements were obtained using aKeithley 236 source measurement unit. Operational lifetime measurementswere performed at room temperature, and devices were aged at variousconstant currents while monitoring their operational voltage and lightoutput. Photoluminescence transients were obtained periodically duringelectrical aging using a time-correlated single photon counting systemfrom Horiba Jobin Yvon, with a wavelength of about λ=335 nm, pulsedexcitation source incident at about 45° from normal. Photoluminescencefrom the emissive layer was obtained at a wavelength of about λ=470 nmto prevent collection of fluorescence from the transport layers.

The current density-voltage (J-V) characteristics and EQE for the OLEDswere plotted in FIGS. 13A and 13B, respectively. The device showed apeak forward viewing EQE=(11.0±0.2) %. The emission spectrum at J=10mA/cm², with a peak at λ=464 nm, was due to the dopant and remained thesame at all current densities, indicating that the recombination zoneremained within the EML. The recombination was highest at the EMLinterface adjacent to the thin mCBP blocking layer 1316. This conclusionwas supported by the lack of NPD emission, and the fact that removal ofthe mCBP blocking layer resulted in significant Alq₃ emission.

FIG. 14A provided the normalized electrophosphorescence versus time forfour different drive current densities: about 6.9, about 15.1, about24.3 and about 34.4 mA/cm² corresponding to initial (t′=0) luminances ofL₀=about 1000, about 2000, about 3000, and about 4000 cd/m²,respectively. The operational lifetime, LT₈₀, corresponded to the timerequired for the luminance to degrade to 0.8 L₀. The rate of luminanceloss increased monotonically with J; here lifetimes decreased from about110 hrs at about 6.9 mA/cm², to about 9 hrs at about 34.4 mA/cm². Thesolid lines in FIG. 14A were derived from the model under the assumptionthat exciton localization on a dopant or host molecule led to defectformation (Equation 4b). The same experimental data were reproduced inFIGS. 14B and 14C to compare the model predictions for exciton-excitonannihilation (Equation 4c) and exciton-polaron (electron) annihilation(Equation 4d) defect formation processes, respectively.

The voltage rise corresponding to the luminance loss was plotted in eachof FIGS. 15A-15C for comparison with each different modeling scenario.The solid lines of FIGS. 15A-15C were calculated using the same excitonlocalization, exciton-exciton, and exciton-polaron degradation models asin FIGS. 14A-14C.

FIGS. 16A-16C showed PL transients obtained from an as-grown device, adevice degraded to a luminance at time t′ of L(t′)=0.59 L_(o)(L₀=1000cd/m²), and one degraded to L(t′)=0.16 L_(o) (L₀=3000 cd/m²). Thepredictions from each model were shown by solid lines in FIG. 16A(exciton localization), FIG. 16B (exciton-exciton annihilation), andFIG. 16C (exciton-polaron annihilation). The as-grown device exhibited anatural decay lifetime of τ=(1.10±0.08) μs, while the degraded devicetransients became increasingly nonlinear, indicative of the existence ofquenching. Fluorescence from NPD overlapping the λ=470 nm detectionwavelength was responsible for the sharp decrease in intensity near t=0.The transients were normalized at the onset of phosphorescence, afterthe fluorescence had decayed to a negligible level (i.e., at about t>0.2μs).

Configurational diagrams of the defect generation mechanisms proposed inEquation (4), were shown in FIG. 17. FIG. 17A shows the excitonlocalization pathway, where a direct or pre-dissociative potential, R,crossed the exciton energy surface. In FIG. 17B, annihilation of twosinglet (S₁) or triplet (T) excitons yielded a ground state (S₀), and anupper excited state (S_(n)* or T_(n)*), which were dissociated via adirect or pre-dissociative reaction (route 1) along R to yield radicalfragments that resulted in defect states. Dissociation also occurred viathe hot-molecule mechanism (route 2) if the upper excited state relaxedvibronically to create a hot first excited state. Similarly, FIG. 17Cshowed annihilation of an exciton (S₁ or T₁) and a polaron (D₀) tocreate a ground state (S₀) and an excited polaron (D_(n)*), whichdissociated along route 1 or 2, analogous to the previous case above.

To determine which of these processes were most active, the data inFIGS. 14-16 to the model discussed above. For each degradation model, asingle set of parameters was used to fit the luminance, voltage, and PLdata. The calculated luminance degradation slope following the ‘knee’(i.e., onset of downward slope) of each curve (FIG. 14) dependedprimarily on the degradation mode assumed, while only slightly on thechoice of parameter values, which determined the position of the ‘knee’in time, t′. Each set of parameters is provided in Table 2, below.

TABLE 2 Model Parameter Values Parameter Exciton Ex-Ex Ex-Pol Variabled_(rec) (nm) 12, 9, 7, 5, ±5 10 ± 3 8 ± 2 K_(DR) (cm³s⁻¹) (4 ± 3) ×10⁻¹² (4 ± 3) × 10⁻¹² (5 ± 3) × 10⁻¹² K_(x)(s⁻¹ or cm³s⁻¹) (6 ± 3) ×10⁻⁶ (1.7 ± 0.9) × 10⁻²² (7 ± 2) × 10⁻²⁴ σ (cm²) 2 × 10⁻¹⁷ 3 × 10⁻¹⁷10⁻¹⁷ E_(t) − E_(Fv) (eV) 0.21 ± 0.05 0.15 ± 0.04 0.17 ± 0.03 Fixed, J(mA/cm²) 6.85 (L₀ = 1000 cd/m²), 15.12 (L₀ = 2000 cd/m²) Common 24.26(L₀ = 3000 cd/m²), 34.36 (L₀ = 4000 cd/m²) τ (μs) 1.10 ± 0.08 μ_(p)(cm²V⁻¹s⁻¹) 2 × 10⁻⁷ μ_(n) (cm²V⁻¹s⁻¹) 8 × 10⁻⁸ T (K) 295 x₁ (nm)  40x₂(nm)  70 x₃(nm) 115

The exciton-polaron model provided the best fit to the data, asdiscussed below. Electron and hole mobilities representative of thosefound for a similar CBP based host-guest combination were kept constantin all fits. In FIG. 14C, the model deviated slightly in advanced stagesof degradation (L(t′)<0.4 L_(o)), where the data showed lower luminancethan predicted. This may result from a change in charge balance due tothe voltage rise FIG. 15. This resulted in higher polaron densities, andthus to an increased rate of degradation, providing a positive feedbacknot considered in the model.

Accordingly, each degradation mechanism was marked by its own, distinct,functional dependence. This was evident from approximate solution ofEquations (1)-(3) in the limit that Q (x,t′) is large (≧10¹⁷ cm⁻³). Useof Equation (4) yielded a polynomial of different order in Q(x,t′) foreach degradation mechanism: quadratic for single polaron localization(Equation (4a)), 4^(th) order for exciton localization (Equation (4b)),7^(th) order for exciton-exciton annihilation (Equation 4(c)), and5^(th) order for exciton-polaron annihilation (Equation 4(d)). Thedistinguishing feature of each degradation mode was thus the order ofthe polynomial used to fit the data, which led to theparameter-independent functional differences in the fits of FIG. 14.

The parameters in Table 1 were consistent with expectations for thisguest-host materials combination. For example, the values suggested thatdefect hole traps are early full, lying at about 0.1 eV above the holequasi-Fermi level. Characteristic recombination lengths were allconsistent with literature reported values of in the range of about 8 nmto about 12 nm. Also, the defect exciton quenching rate, K_(DR)˜4×10⁻¹²cm³s⁻¹, was similar to that reported for other bimolecular quenchingreactions in OLEDs. Low capture cross-sections of about σ=10⁻¹⁷ cm⁻³resulted from localization of large effective mass holes that werecharacteristic of organic molecules.

The relative contributions to luminance loss from defect excitonquenching and non-radiative recombination were estimated to be about 70%and about 30% respectively. The existence of quenching was confirmed bythe PL data of FIG. 16, and non-radiative recombination is inferred fromthe presence of charged defects that led to the observed voltage rise.

The average defect density, Q_(AVG)(t′)=1/d_(rec)∫Q(x,t′)dx, wascalculated using the exciton-polaron model, is shown in 18A. Theincrease in defect density was linear for t<10 hrs, and rolls off atlonger times. From FIG. 14C and FIG. 18A, it was inferred that a defectdensity of about 10¹⁸ cm⁻³, or about 0.1% of the molecular density leadsto greater than about 50% loss in luminescence. The rates of defectformation, F_(X)(t′), corresponding to the densities in FIG. 18A, wereplotted in FIG. 18B. At about 1000 cd/m², F_(X)≈0.04, or about 1 defectper about 25 excitons was formed every hour.

The effects of exciton-polaron annihilation on device lifetime may bereduced by increasing d_(rec) and decreasing K_(X), as shown in FIGS.18A and 18B, respectively. These results were calculated for a devicewith L₀=1000 cd/m², maintaining all other parameters at the values inTable 1. The device lifetime more than doubled when recombination wasuniform across the EML (d_(rec)→∞), as compared to d_(rec)=8 nm foundfor the devices studied here. Further, for d_(rec)>30 nm, there waslittle improvement in LT₈₀. Since the voltage of an OLED may be stronglydependent on layer thickness, about 30 nm may be considered a nominalupper limit to the EML thickness in practical, efficient OLEDs usingsome criteria. However, fabricated devices having thick EML showedremarkable lifetimes, as explained elsewhere in this application, sothicker EML may be useful in many instances. Also, in FIG. 18B, a 6-foldincrease of LT₈₀ was calculated as K_(X) was reduced from about 7×10⁻²⁴cm⁻³s⁻¹, to about 1×10⁻²⁴ cm⁻³s⁻¹.

As indicated in FIG. 17C, it was the excess energy gained by the polaronthat drove the exciton-polaron degradation mechanism. It enabled directand pre-dissociation reactions (route 1) if the repulsive potential, R,existed. However, due to the typically fast (ps) rates of vibronicrelaxation from upper excited states, the hot-molecule mechanism (route2) was perhaps even more important. In this case, vibrationaldissipation of the excess electronic energy (about 2.7 eV) led tocleavage of low energy molecular bonds.

Guest triplet excitons and host polarons were likely to be the dominantparticipants in the exciton-polaron defect formation reactions. Theguest exciton density was much higher than the density on the host,since lifetimes on the host were short (less than about 1 ns) due torapid energy transfer to the guests, where they exist as triplets forabout 1 μs. Since both FOrster and exchange exciton-polaron annihilationmechanisms were strongly distance dependent, the physical separation ofguests discouraged guest-guest annihilations. Accordingly, it wasinferred that energy exchanged by annihilation of the guest tripletexciton to the host polaron resulted in a dissociative process of thehost molecule itself. The fragments were in close proximity to the guestmolecule and thus quenched any subsequent triplets on that molecule,rendering it a permanent non-radiative center. Furthermore, as notedabove, the fragmented molecule also acted as a deep trapping center,resulting in the observed operating voltage rise.

It is apparent that the efficiency of hot-molecule dissociation mayincrease with the amount of energy transferred to the polaron. Sincethis energy was provided by the guest, this suggested that thedegradation rate (K_(x)) was a function of the guest exciton energy. Inthis case, red phosphorescent OLEDs would show the longest lifetimes,followed by green and then by blue devices. This has been observed inall OLED reliability studies to date in both polymer and small molecularweight systems, as well as for electrofluorscent andelectrophosphorescent guest-host materials combinations. For example,the longest reliability observed in the red, green and blue Ir-basedsmall molecule electroospsphorescent OLEDs were about 10⁶ hrs, 5×10⁴ hrsand about 2×10⁴ hrs, respectively. Although there were significantdifferences between the device structures and test conditions used ineach of these studies, the lifetime scaling was clearly apparent and wasconsistent with our energy-based model for device degradation.

Strategies for minimizing exciton-polaron annihilation involved loweringeither K_(x) or the densities of excitons and polarons. FIG. 19A showsthe results of lowering both exciton and polaron densities by expandingthe recombination zone. For example, control of electron and holemobilities in the EML as well as the strategic placement of energybarriers in the device may lead to a more uniform and distributedrecombination zone. Finally, engineering host and guest molecules tolower the annihilation probability may lead to increased lifetime (FIG.19B) as well as improved efficiency at high brightness. Due to thedistance dependence of annihilation processes, increasing theintermolecular separation through addition of steric bulk to guest andhost molecules may lead to decreased K_(x) and longer lifetimes,although this may also reduce the device efficiency by impeding excitonor charge transfer within the EML.

Specific Example 2

Using a conventional vacuum thermal evaporator, about a 50 nm film ofmCBP containing about 6 wt. % emitter C was deposited onto a glasssubstrate. The film was moved directly from the vacuum evaporator into anitrogen glove box with fluorescent white lamps. Oxygen was then bledinto the glove box until it was present at 50 ppm, then the film wasencapsulated using a glass lid and an epoxy seal. More oxygen was bledin to the box until 500 ppm of it was present; then, another film wasencapsulated. Finally, 5000 ppm of oxygen was admitted into the box anda third film was encapsulated. By controlling the amount of oxygen inthe box, the photoluminescence of the films was changed as shown in FIG.20. More oxygen produces more red emission relative to blue emission ofthe deposited dopant. FIG. 20 shows the relative photoluminescencecontributions of the blue emission with peaks at about 460 nm and about490 nm and the orange emission with peak at about 570 nm were controlledby changing the amount of oxygen to which the film was exposed.

While the invention is described with respect to particular examples andspecific embodiments, it is understood that the present invention is notlimited to these examples and embodiments. For example, thephosphorescent materials may contain stereo and/or structural isomers.The invention as claimed therefore includes variations from theparticular examples and specific embodiments described herein, as willbe apparent to one of skill in the art.

Specific Example 3

A device was fabricated where each layer has been sequentially depositedusing standard methods. For each example, unless otherwise described,the standard method involved purchasing substrates coated with ITO fromNippon Sheet Glass Co. Ltd of Kanagawa-ken, Japan. All layers subsequentto the ITO were deposited by vacuum thermal evaporation (VTE). Thedevice of specific example 3 was composed of the following sequentiallayers: an ITO anode having a thickness of about 80 nm, a layer ofLG101™ (purchased from LG, Korea) having a thickness of about 10 nm, anNPD layer having a thickness of about 30 nm, a mCBP layer including9/12/15% dopant A, a mCBP layer having thickness of about 5 nm, a ALQ₃layer having a thickness of about 30 nm, and a LiF/Al cathode (See FIG.3, Panel I).

Specific Example 4

A device was fabricated where each layer has been sequentially depositedusing standard methods. The device was composed of the followingsequential layers: an ITO anode having a thickness of about 80 nm, amCBP layer including 15% dopant B having a thickness of about 70 nm, amCBP layer having thickness of about 10 nm, a ALQ₃ layer having athickness of about 30 nm, and a LiF/Al cathode (See FIG. 4, Panel I).

Specific Example 5

A device was fabricated where each layer has been sequentially depositedusing standard methods. The device was composed of the followingsequential layers: an ITO anode having a thickness of about 80 nm,Ir(ppy)₃ layer having a thickness of about 10 nm, a mCBP layer including15% dopant B having a thickness of about 70 nm, a mCBP layer havingthickness of about 10 nm, a Alq₃ layer having a thickness of about 30nm, and a LiF/Al cathode (See FIG. 5, Panel I).

Specific Example 6

A device was fabricated where each layer has been sequentially depositedusing standard methods. The device was composed of the followingsequential layers; an ITO anode having a thickness of about 80 nm,Ir(ppy)₃ layer having a thickness of about 10 nm, a NPD layer having athickness of about 30 nm, a mCBP layer having thickness of about 30 nm,a Alq₃ layer having a thickness of about 40 nm, and a LIF/Al cathode(See FIG. 6, Panel I).

It is understood that the various embodiments described herein are byway of example only, and are not intended to limit the scope of theinvention. For example, many of the materials and structures describedherein may be substituted with other materials and structures withoutdeviating from the spirit of the invention. It is understood thatvarious theories as to why the invention works are not intended to belimiting.

1-10. (canceled)
 11. A method of depositing an emissive layer for anorganic light emitting device, comprising: providing a source of aphosphorescent emissive dopant having a peak emissive wavelength lessthan 500 nm, wherein the source has a purity in excess of 99.5% asdetermined by high pressure liquid chromatography; the source furtherhas a combined concentration of halide and metal impurities below 100ppm; the source being sufficiently pure that a residue corresponding toless than 5 wt % of the original charge in the sublimation crucibleremains after the original charge is fully depleted; and thephosphorescent emissive dopant has a sublimation temperature less than350° C. and is deposited via sublimation; depositing the phosphorescentemissive dopant via sublimation, along with a host.
 12. The method ofclaim 11, wherein the phosphorescent emissive dopant is deposited in anenvironment having less than 10 ppm oxygen, and in the near absence oflight having a wavelength less than 600 nm.
 13. The method of claim 11,wherein the phosphorescent emissive dopant is deposited in a vacuumsystem having a pressure level less than 1×10-8 Torr.
 14. The method ofclaim 11, wherein the phosphorescent emissive dopant is deposited in avacuum system having a pressure level in the range of about 1×10-8 Torrto 1×10-12 Torr.
 15. The method of claim 11, wherein the phosphorescentemissive dopant is deposited in a vacuum system having a pressure levelin the range of about 1×10-9 Torr to 1×10-12 Torr.
 16. The method ofclaim 11, wherein the phosphorescent emissive dopant is deposited in avacuum system having a pressure level in the range of about 1×10-10 Torrto 1×10-12 Torr.
 17. The method of claim 12, wherein the phosphorescentemissive dopant comprises a cyclometallated N, C-donorimidazophenanthridine ligand comprising at least one 2,2,2-trialklethylsubstituent. 18-26. (canceled)
 27. The method of claim 15, wherein thephosphorescent emissive dopant is deposited in a vacuum system having apressure level in the range of about 1×10⁻⁸ Torr to about 1×10⁻¹² Torr.28-36. (canceled)